Warning! Thread Necromancy!
I recently updated a tube fin rocket with the addition of rail buttons in place of a large launch lug.
Lowe's does indeed stock 6-32 t-nuts in their fastener section and they are usually in the drawers of parts. I prefer the t-nuts that are labeled as "brad hole" because they do not have the bent up teeth to keep the part from rotating. A dab of epoxy will hold the t-nut in place.
The 6-32 t-nuts seem to require a 3/16" hole drilled in the body tube. (It goes without saying that the t-nut goes inside the fuselage/body tube and the rail button goes on the outside so I won't say that.) Bend the t-nut flange slightly to make it fit the curve of the body tube. Make sure the screw through the rail-button does not protrude into the body tube where it might snag the recovery system. Choose an appropriate sized fastener or cut to size.
For those wondering how to launch a tube fin rocket from a rail, the rail passes through one of the tube fins and a pair of stand-offs for the rail buttons are used to provide clearance between the fin and the rail. Stand-offs can be made or purchased. I buy mine from Dog House Rocketry (now owned by Mike Fisher). It was necessary to clamp the stand-offs into a vise and drill the holes out to 3/16" to fit the t-nuts. Another solution might be to stack two rail buttons and use only the top button. I chose not to go this route because of the possibility of a mismount at the pad. Excitable boy, ya know?